1. Field of the Invention
The present invention relates to a method of forming bumps used for forming conductive bumps on electronic parts. Also, the present invention relates to a method and an apparatus of fabricating a perforated plate which can be used as a template.
2. Description of the Related Art
A method of forming metallic bumps such as solder bumps on an electronic part such as an LSI and joining the electronic part to a printed circuit board by the metallic bumps has come into wide use. In one method of forming the metallic bumps, metallic balls are previously formed and stored in a metallic ball container, and then the metallic balls are taken out from the metallic ball container and attached to the electrodes of the electronic part so that the metallic balls can be used as metallic bumps of the electronic part. For example, Japanese Unexamined Patent publication No. 8-112671 discloses the above-described metallic bump forming apparatus.
In the conventional metallic bump forming apparatus, a suction head is used to take out the metallic balls from the metallic ball container and attach them to the electrodes of electronic part. A template (suction pad) for mounting metallic balls is attached to the suction head, the template having through-holes arranged in the same arrangement as that of the electrodes of the electronic part. Suction grooves are provided in the surface of the sucking head and are connected to a vacuum source, and the through-holes of the template for mounting metallic balls are connected to the suction grooves of the suction head, so that the metallic balls can be attracted by vacuum suction.
When the template for mounting metallic balls comes into contact with the metallic balls accommodated in the metallic ball container, the metallic balls are respectively sucked by and attracted to the through-holes in the template. When the suction head is then moved above the electronic part, the metallic balls attracted to the through-holes in the template are respectively located on the electrodes of the electronic part. Then, the metallic balls are joined to the electrodes of the electronic part under heat and pressure, the metallic balls becoming the metallic bumps. After that, the suction head is separated from the electronic part, with the result that the metallic balls are transferred from the suction head to the electronic part.
The template for mounting metallic balls is usually made of inorganic material such as glass, which is resistant to distortion, and its charging level is low. Plastics are not suitable for the material for the template for mounting metallic balls, because plastics are easily distorted, and their charging level is high. If the template for mounting metallic balls is made of a plate which is easily distorted, the template is distorted and the positions of the through-holes are shifted, from the positions of the electrodes of the electronic parts, when the metallic balls are bonded to the electrodes of the electronic part under heat and pressure by means of thermo-compression. When the template for mounting metallic balls has an electrification property, the metallic balls are attracted to the template at positions other than those of the through-holes by static electricity when the template is moved above and then into the metallic ball container to suck the metallic balls to the through-holes.
Through-holes in the template for mounting metallic balls are formed by etching. Alternatively, the through-holes can be formed by machining such as drilling or electric discharge machining. However, when the template for mounting metallic balls is made of inorganic material such as glass, the side walls of the through-holes become unsmooth, and there is a possibility of generation of burrs which are irregularities of the side walls of the through-holes.
If the side walls of the through-holes are not smooth, the following problems may be caused. The metallic balls bite into the end portions of the through-holes in the template for mounting metallic balls when the template is pressed against the electronic part to join the metallic balls to the electrodes of the electronic part, and the metallic balls adhere to the template and the metallic balls are torn off from the electrodes of the electronic part, when the suction head is separated from the electronic part.